Oxygen masks are well known in the art as a tool for fighting fires in an enclosed structure. A portable oxygen mask that can provide a steady and controlled stream of oxygen while maintaining a weight that allows for freedom of movement is a necessity when fighting fire. This need is never more prevalent than in the confined and pressurized environment of an aircraft. An aircraft fire presents many additional dangers due to its pressurized compartments and the presence of oxygen in large quantities. Therefore, there is a need for a reliable and compact oxygen mask that is light weight and well suited for all closed environments, particularly those of an aircraft.
The Protective Breathing Equipment (PBE) is a closed circuit breathing apparatus designed to help protect the wearer's eyes and respiratory tract in an atmosphere containing smoke and fumes by isolating the eyes and breathing functions from the environment. Isolation is achieved by a hood system that envelops the head of the wearer. A breathable atmosphere is maintained by a demand-based chemical air regeneration system that supplies oxygen and removes carbon dioxide and water vapor. This equipment is certified in accordance with the requirements of TSO-C116.
The PBE is a hood device that completely encloses the head of the wearer and seals at the neck with a thin elastic membrane. The large internal volume of the hood accommodates glasses and long hair while the elastic membrane neckseal enables fitting over the broad population range representative of aircraft crewmembers. The chemical air regeneration system is based on the use of potassium superoxide (KO2). Operation of the PBE is silently and reliably powered by the exhalation of the wearer into an oronasal mask cone located within the hood. The low moisture content of the oxygen gas generated by the KO2 bed in the canister reduces the wet bulb temperature, improves wearer comfort, and controls misting or fogging of the visor, side windows, and/or glasses. The complete device is secured to the head to minimize restrictions to mobility. The large optically clear visor and side windows provide a wide field of vision while maintaining their relative position with the head. A neck shield extends downward from the back of the hood to protect the collar and upper shoulder area of the user from direct flame contact. A speaking diaphragm is installed in the oronasal mask cone to enhance communication.
Protective breathing apparatus (PBE) for use on aircraft are stored in sealed bags to ensure that they are free of moisture and carbon dioxide. When the device is needed, it is removed from its storage location and the sealed bag is opened. The user then deploys the PBE over his or her head and shoulders and initiates the oxygen generation unit. An exemplary PBE is shown in FIG. 1. During operation, the user exhales into the oronasal mouthpiece. The exhaled breath travels through an exhalation duct and enters a canister containing KO2 (potassium superoxide). The exhaled carbon dioxide and water vapor are absorbed and replacement oxygen is released according to the reaction below:2KO2+H2O→2KOH+1.5O2 2KO2+CO2→K2CO3+1.5O2  Oxygen Generation:2KOH +CO2→K2CO3+H2OKOH+CO2→KHCO3  Carbon Dioxide Removal:The regenerated oxygen gas passes through the inhalation duct and enters the main compartment, or breathing chamber, of the PBE hood. The interior hood volume above the neck seal membrane serves as the breathing chamber. When the user inhales, the one-way inhalation valve allows the regenerated gas to enter the oronasal mouthpiece and thus travel to the respiratory tract of the user. The breathing cycle can continue in this manner until the KO2 canister is exhausted.
In the event of a fire on the aircraft, the PBE is removed from storage and is quickly transitioned from a vacuum environment inside its storage bag to the nominal environment of the aircraft cabin. The rapid pressure increase can affect the components of the PBE, and in particular can stretch, deform, or rupture the exhalation duct. That is, while the canister is still largely in the predominantly vacuum environment of its storage, the pressure differential between the canister and the outside is nil. However, once the bag is opened, a large pressure differential across the diaphragm can be created by the ambient pressure outside and the vacuum inside. This pressure differential across the membrane can draw the inhalation duct into the canister, leading to stretching, tearing, and deformation. Any of this type of damage to the exhalation duct can significantly reduce the duration of the PBE's effectiveness.